Diamond Shape Rounded
Final:



Steps:




Subdivide a Plane (W > Subdivide) as much as you need, keep in mind to have vertex in the center of a Plane. So you will need odd number.
Select vertex in the center.
Turn on Proportional Editing (you can switch trough different falloffs to get different results).
Scale down (S) and increase Proportional Editing influence size - MouseWheel.